Plasmodium vivax is the most common cause of malaria outside of Africa and a serious economic and health burden in many developing countries (Mendis et al., (2001) Am. J. Trop. Med. Hyg. 64: 97-106). Vivax malaria is often a severe debilitating disease in adults as well as children with an impact and occurrence that is underreported (Price et al., (2007) Am. J. Trop. Med. Hyg. 77: 79-87; Mueller et al., (2009) Lancet Infect. Dis. 9: 555-566). Diagnosis is more difficult than for P. falciparum since the parasite selectively invades reticulocytes thereby leading to lower parasitemias and lower level transmission intensity, which in turn is believed to lead to development of weak protective immunity by intermittent exposure of individuals living in these endemic areas (Mendis et al., (2001) Am. J. Trop. Med. Hyg. 64: 97-106; Mueller et al., (2009) Lancet Infect. Dis. 9: 555-566; Chootong et al., (2010) Infect. Immun. 78: 1089-1095). The biology and mechanism of transmission therefore gives this parasite great resilience while increasing the need for new approaches for control and management of P. vivax infections.
There is enough evidence suggesting a blood-stage vaccine should be part of the overall strategy for malaria control (Arevalo-Herrera et al., (2010) Hum. Vaccines 6: 124-132). Similar to other species, morbidity caused by P. vivax is associated with the asexual blood-stage growth although severe clinical disease typically occurs at much lower parasitemias. Therefore, a vaccine based on these stages will help to reduce or eliminate clinical manifestations observed during vivax malaria. Merozoite proteins, which play a major role during the invasion process, are important candidates for vaccine development aimed at neutralizing blood-stage growth. Progress in the identification, characterization and development of these vaccine candidates, as well as all new prophylactic and therapeutic control measures against vivax malaria, have been seriously hindered by lack of a long-term P. vivax in vitro culture system (Udomsangpetch et al., (2008) Trends Parasitol. 24: 85-88). The Duffy binding protein (DBPII) is one of the few merozoite proteins that have been well characterized with a critical role in P. vivax blood-stage development.
Malaria parasites depend on specific receptor-ligand interactions for successful invasion of the host erythrocytes. In P. vivax, blood-stage development depends on DBPII interaction with the Duffy Antigen Receptor of Chemokines (DARC) on human erythrocytes for erythrocyte invasion, a process mediated by the PvDBPII and PkDBPII respectively (Miller et al., (1975) Science 189: 561-563; Barnwell et al., (1989) J. Exp. Med. 169: 1795-1802; Wertheimer & Barnwell (1989) Exp. Parasitol. 69: 340-350). It has been suggested that this molecule plays an important role during the process of junction formation just before invasion (Adams et al., (1990) Cell 63: 141-153). Individuals lacking DARC on their erythrocyte have been shown to be refractory to P. vivax infection (Miller et al., (1976) New Eng.l J. Med. 295: 302-304). This dependence of P. vivax on DBPP for invasion makes DBPII a prime target for vaccine development against vivax malaria. PvDBPII is a member of the erythrocyte binding proteins (EBPs) family, characterized by highly conserved cysteine-rich domains known as the region II (Adams et al., (1992) Proc. Natl. Acad. Sci. U.S.A. 89: 7085-7089; Chitnis & Miller (1994) J. Exp. Med. 180: 497-506; Sim et al., (1994) Science 264: 1941-1944). In PvDBPII, this region, (DBPII) also known as the ligand domain contains the residues critical for receptor recognition (VanBuskirk et al., (2004) Proc. Natl. Acad. Sci. U.S.A. 101: 15754-15759; Chitnis et al., (1996) J. Exp. Med. 184: 1531-1536; Ranjan & Chitnis (1999) Proc. Natl. Acad. Sci. U.S.A. 96: 14067-14072).
Individuals in endemic regions are known to develop anti-DBPII antibodies with significant quantitative and qualitative differences in their serological responses (Chootong et al., (2010) Infect. Immun. 78: 1089-1095; Fraser et al., (1997) Infect. Immun. 65: 2772-2777; Xainli et al. (2002) J. Immunol. 169: 3200-3207; Xainli et al., (2003) Infect. Immun. 71: 2508-2515; Cole-Tobian et al. (2002) J. Infect. Dis. 186: 531-539) and this response is known to increase with age as a result of boosting effect due to recurrent exposure (Grimberg et al., (2007) PLoS Med 4: e337; Michon et al., (2000) Infect. Immun. 68: 3164-3171; King et al., (2008) Proc. Natl. Acad. Sci. U.S.A. 105: 8363-8368). Naturally acquired antibodies to the Plasmodium vivax DBPII have been shown to inhibit binding of DBPII to DARC on human erythrocyte (Chootong et al., (2010) Infect. Immun. 78: 1089-1095; Grimberg et al., (2007) PLoS Med 4: e337; Michon et al., (2000) Infect. Immun. 68: 3164-3171; Dutta et al., (2000) Mol. Biochem. Parasitol. 109: 179-184). Furthermore, it has been demonstrated that antibodies raised against rDBPII and rPkDBPII which is a homolog of DBPII (greater than 70% identity) can inhibit erythrocyte invasion by both P. vivax (Michon et al., (2000) Infect. Immun. 68: 3164-3171) and P. knowlesi (Singh et al., (2002) Mol. Biochem. Parasitol. 121: 21-31) as well as block cytoadherence of DBPII to human erythrocytes (Yazdani et al., (2004) Biotechnol. Lett. 26: 1891-1895). These data gives further support for DBPII as a candidate for the development of a vaccine against P. vivax malaria. DBPII contains a large number of polymorphisms, thought to be exerted by host immune response, a mechanism suggested to be used by the parasite for immune evasion (Xainli et al. (2002) J. Immunol. 169: 3200-3207; Tsuboi et al. (1994) Infect. Immun. 62: 5581-5586; Cole-Tobian & King (2003) Mol. Biochem. Parasitol. 127: 121-132). These polymorphisms influence the immunogenicity of DBPII leading to strain-specific immunity in P. vivax (King et al. (2008) Proc. Natl. Acad. Sci. U.S.A. 105: 8363-8368; VanBuskirk et al. (2004) J. Infect. Dis. 190: 1556-1562; Ceravolo et al., (2009) Clin. Exp. Immunol. 156: 502-510).
Plasmodium vivax dependence on binding of the P. vivax Duffy-binding protein (PvDBPII) to the erythrocyte Duffy antigen receptor (Duffy antigen receptor for chemokines, DARC) for invasion into the cell, makes this interaction an attractive target for intervention (Adams et al., (1990) Cell 63: 141-153; Adams et al., (1992) Proc. Natl. Acad. Sci. U.S.A. 89: 7085-7089; Chitnis & Miller (1994) J. Exp. Med. 180: 497-506; Haynes et al., (1998) J. Exp. Med. 167:1873-1881; Miller et al., (1976) New England J. Med. 295: 302-304; Wertheimer & Barnwell (1989) Exp. Parasitol. 69: 340-350). The PvDBPII recognition site for the erythrocyte receptor has been mapped to an area between cysteines 4 and 7 of the DBL domain (Chitnis et al., (1996) J. Exp. Med. 184:1531; Ranjan & Chitnis (1999) Proc. Natl. Acad. Sci. USA 96:14067; VanBuskirk et al., (2004) Proc. Natl. Acad. Sci. USA 101:15754; Singh et al., (2003) Biochem. 374:193). This is the most highly polymorphic region of the open reading frame with a high rate of nonsynonymous to synonymous polymorphisms, suggesting positive selection indicative of immune pressure (Ampudia et al., (1996) Mol Biochem Parasitol 78:269; Cole-Tobian & King (2003) Mol. Biochem. Parasitol. 127:121; Cole-Tobian et al., (2002) J. Infect. Dis. 186:531; Kho et al., (2001) Korean J. Parasitol. 39:143; Tsuboi et al., (1994) Infection and Immunity 62:5581; Xainli et al., (2000) Mol. Biochem. Parasitol. 111:253). The non-homologous proteins Influenza hemagglutinin (HA) and Plasmodium apical membrane antigen 1 (AMA-1) reveal a pattern of polymorphisms located adjacent to and surrounding their receptor binding sites. A consensus viewpoint interprets these substitutions as making it more difficult for host inhibitory antibodies, elicited by previous exposure to the pathogen, to recognize new variant epitopes and block the interaction between the pathogen ligand and the host receptor (Bai et al., (2005) Proc. Natl. Acad. Sci. USA 102:12736; Coley et al., (2006) Infection and Immunity 74:2628; Crewther, et al., (1996) Infection and Immunity 64:3310; Healer et al., (2004) Mol. Microbiol. 52:159; Pizarro et al., (2005) Science 308:408; Smith et al., (2004) Science 305:371; Wilson & Cox (1990) Annu. Rev. Immunol. 8:737)